A permanent magnet embedded within a rotor of an IPM motor, etc., is required to have a coercivity that makes it possible to resist demagnetization caused by an external magnetic field coming from the stator core side.
The external magnetic field acting on this permanent ma net is generally such that, with respect to a plan view of the rotor in which the permanent magnet is embedded, it is greatest at the corner parts on the stator core-side of the permanent magnet, while becoming smaller towards the center of the rotor core.
On the other hand, with respect to sintered permanent magnets, metal particles for enhancing the coercivity performance of the permanent magnet are diffused along the grain boundary from the surface thereof, and so forth. However, since rare metals, such as dysprosium, terbium, etc., are used for these metal particles, one important problem to be solved in the art from the perspective of manufacturing cost reduction for permanent magnets is how to reduce the used amount thereof while at the same time securing the desired coercivity performance.
With respect to this coercivity performance, since, as mentioned above, the magnitude of the external magnetic field that acts on a permanent magnet varies from part to part, the coercivity required of the permanent magnet also differs from part to part. Thus, with a view also to reducing the amount of rare metal used to enhance the coercivity performance, by producing a coercivity distribution magnet whose coercivity of the permanent magnet varies from part to part (i.e., having a coercivity distribution), it is possible to realize permanent magnet production wherein the amount of rare metal, e.g., dysprosium, etc., used is reduced as much as possible while meeting the required coercivity performance, and in which a reduction in production cost is achieved.
If a coercivity distribution magnet is produced through, by way of example, grain boundary diffusion, it is important that it be checked whether or not each part of this coercivity distribution magnet has the required coercivity (coercivity that varies from part to part) to guarantee product reliability.
However, it may readily be appreciated that since the coercivity is distributed, determining whether or not each part has the required coercivity is difficult and involves a time-consuming checking process.
It is noted that thus determining whether or not each part of a coercivity distribution magnet has the required coercivity is performed mainly in a quality checking stage before producing and shipping the coercivity distribution magnet.
Further, it is preferable that, in this quality checking stage, a determination as to whether or not each part of the coercivity distribution magnet has a coercivity that is equal to or greater than the required coercivity be performed in a short period of time and as inexpensively as possible.
There is disclosed in Patent Literature 1 a testing device for measuring a demagnetization state of a motor magnet, the device comprising at least: a jig that holds a motor, which comprises a stator and a rotor that is located therewithin and comprises a magnet, in such an attitude that the stator and the rotor have a given mechanical angle; a power source that passes a direct current through a coil wound around the stator; and a flux meter that measures a demagnetization state of the magnet. By employing this testing device, it is possible, without using an actual vehicle for example, to reproduce the desired temperature conditions, mechanical angle of the motor (in the case of an IPM motor, the rotation angle of a given permanent magnet with respect to a given tooth), and conduction conditions (in the case of a three-phase AC motor, the conduction states of each phase, namely U, V, and W, at a given moment). And it becomes possible to evaluate the demagnetization mode of a magnet (permanent magnet) under those conditions (e.g., how much demagnetization has occurred in which part of the magnet, how large the magnetic flux of the magnet as a whole is, etc.).